1. Field of the Invention
The present invention relates to a magnetic encoder in which harmonic distortion can be reduced and a harmonic distortion reducing method for the magnetic encoder.
2. Description of the Related Art
Utilizing a magnetic encoder as an angle measuring device is known in the art. Inventions in which a magnetic encoder is used as an angle measuring device for a conventional surveying instrument, such as a total station or a theodolite have been proposed. In general, a magnetic encoder includes a magnetic drum which rotates together with a collimating telescope and a magnetic sensor which detects the angular displacement of the magnetic drum. The magnetic drum is provided on its outer peripheral surface with a multipolar-magnetized layer having divided magnetized portions which are spaced at equal pitches corresponding to a dividing number N (N is a positive integer). The magnetic sensor is opposed to the multipolar-magnetized layer. The magnetic sensor is provided with, for example, four magnetic reluctance elements which are spaced at pitches smaller than the pitches of the divided magnetized portions. Therefore, the angular displacement of the magnetic drum is measured at the pitches of the magnetized portions by detecting the resistances of the magnetic reluctance elements which are varied in accordance with the rotation of the magnetic drum. The rotation angle of the magnetic drum between the pitches can be measured by interpolation.
However, in the case of a magnetic encoder, the dividing number of the magnetized portions of the multipolar-magnetized layer cannot be increased to that of an optical encoder, and furthermore, the pitch is large. Consequently, influence of distortion in one pitch (i.e., harmonic distortion) is large, due to dimensional error and deviation from the ideal magnetic reluctance curve of the magnetic reluctance elements.
The present invention provides a magnetic encoder, which can be suitably used in surveying, in which harmonic distortion of an arbitrary order is reduced, and also provides a harmonic distortion reducing method thereof.
In the specification, the phrase xe2x80x9creducing harmonic distortionxe2x80x9d refers to the reduction of the influence of harmonic distortion, and/or the meaning thereof also includes the reduction of error caused by harmonic distortion.
For example, an embodiment includes a magnetic encoder including a magnetic drum which is provided on the outer peripheral surface thereof with a plurality of magnetized portions which are spaced at equal pitches; a magnetic sensor unit which is opposed to the outer peripheral surface of the magnetic drum, wherein one of the magnetic drum and the magnetic sensor constitute a rotational member, the rotational member being relatively rotatable with respect to the other of the magnetic drum and the magnetic sensor, wherein output detection signals from the magnetic sensor unit are cyclically varied in accordance with the relative rotation angle of the rotational member and the other of the magnetic drum and the magnetic sensor; a plurality of magnetic sensors which are provided in the magnetic sensor unit, wherein in order to correct harmonic distortion of an arbitrary order, the magnetic sensors are spaced at a distance smaller than one pitch of the magnetized portions, so that detection signals of each of the magnetic sensors are offset by a predetermined phase difference; a calculation device for calculating two sets of vectorial signals, from detection-signal output levels of the plurality of magnetic sensors generated upon rotation of the rotational member, using the following equations (1) and (2):
xe2x80x83Aout=xcexa3Wm=1Vmxc3x97sin xcex8mxe2x80x83xe2x80x83(1); and
Bout=xcexa3Wm=1Vmxc3x97cos xcex8mxe2x80x83xe2x80x83(2); 
wherein W designates the number of the plurality of magnetic sensors; Vm designates the detection-signal output levels of the plurality of magnetic sensors generated upon rotation of the rotational member; Aout designates a set of vectorial signals; and Bout designates another set of vectorial signals. According to this construction, harmonic distortion of an arbitrary order can be reduced.
It is desirable for the calculation device to calculate a rotation angle of the rotational member by an interpolation calculation of the two sets of vectorial signals.
A method is provided for reducing harmonic distortion for a magnetic encoder, including a rotational member which is provided on the outer peripheral surface thereof with a plurality of magnetized portions which are spaced at equal pitches, and a plurality of magnetic sensors which are opposed to the outer peripheral surface of the rotational member, wherein output detection signals from the plurality of magnetic sensors are cyclically varied in accordance with the rotation angle of the rotational member. The method includes arranging the plurality of magnetic sensors so as to be spaced at a distance smaller than one pitch of the magnetized portions, so that detection signals of the respective magnetic reluctance elements are offset by a predetermined phase difference in order to correct harmonic distortion of an arbitrary order; and substituting output levels of the detection signals of the plurality of magnetic sensors, wherein the detection signals are detected when the rotational member rotates, into the following equations (1) and (2):
Aout=xcexa3Wm=1Vmxc3x97sin xcex8mxe2x80x83xe2x80x83(1); and 
Bout=xcexa3Wm=1Vmxc3x97cos xcex8mxe2x80x83xe2x80x83(2); 
wherein W designates the number of the plurality of magnetic sensors; Vm designates the detection-signal output levels of the plurality of magnetic sensors generated upon rotation of the rotational member; Aout designates a set of vectorial signals; and Bout designates another set of vectorial signals.
A rotation angle of the rotational member can determined by an interpolation calculation of the two sets of vectorial signals.
The interpolation calculation can be performed using tanxe2x88x921 (Aout/Bout).
It is desirable for each of the plurality of magnetic sensors to be a magnetic reluctance element, wherein the magnetic reluctance elements are provided on a single substrate.
The output signals of the magnetic reluctance elements can be A/D-converted so as to produce digital signals, so that the interpolation calculation is carried out by the equations (1) and (2) on the digital signals.
The A-D converted digital signals can be corrected using a predetermined coefficient.
Alternatively, the output signals that have the same coefficient in the equations (1) and (2) can be added by an analog process, wherein the added output signals are thereafter A/D-converted.
The analog process can include an electrical connection of corresponding magnetic reluctance elements in series.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-28185 (filed on Feb. 5, 2001) which is expressly incorporated herein by reference in its entirety.